B1901 is Toyota's code for an open circuit in the front seat belt pretensioner squib on the right side of the vehicle. Toyota prints it as B1901/73. The center airbag sensor assembly sent current down that loop, got no return, and stored the code. On a left hand drive vehicle the right side is the front passenger belt.
Toyota builds the front pretensioner into the belt assembly itself. Service data lists the front pretensioner squib RH circuit as consisting of the center airbag sensor assembly and the front seat outer belt assembly RH. The squib is the initiator inside that belt assembly, and the module watches its resistance the same way it watches an airbag inflator.
Four numbers share this circuit, and the last digit tells you the failure mode. B1900/73 is a line short, B1901/73 is an open, B1902/73 is a short to ground, and B1903/73 is a short to B positive. The left side uses B1906 for the same open condition. Reading the exact number saves guessing, because an open and a short send you to opposite ends of the same harness.
Toyota lists three trouble areas for this code and no others: the No. 2 floor wire, the front pretensioner squib RH inside the belt assembly, and the center airbag sensor assembly. Work them in that order and the diagnosis stays short.
Circuit, and the most common reason it goes open is hardware that already did its job. A pretensioner fires once. After that the squib is spent and the loop reads open for good, so B1901 turns up constantly on vehicles that went through a front collision and had the airbags replaced while the belts were left alone. In that case the code sits next to stored deployment data and both have to be dealt with. On a vehicle with no collision history, the same code points at the floor wire or a connector.
No. The module rechecks the loop at every key on, so clearing memory while the squib is spent or the wire is broken accomplishes nothing. The belt has to be made right first. A fired pretensioner is repairable: our seat belt pretensioner repair rebuilds the fired unit and returns the assembly working, which restores the circuit the module is looking for. Once the belts and any deployed airbags are handled, an airbag module reset clears the crash data out of your original center airbag sensor assembly.
B1901 in this form belongs to Toyota and to Lexus and Scion vehicles built on the same restraint charts, since all three share the center airbag sensor assembly naming and the slash subcode format. Toyota assigns the 1900 block to belt pretensioner squibs, which is worth remembering when a code list gives you nothing but the number. Everything above B0999 is manufacturer defined, so a B1901 pulled from another brand is a different circuit entirely and none of this applies to it.
Confirm the pretensioner has not fired before doing any electrical work. A fired belt often looks normal from the driver seat, so check whether the webbing pulls and retracts freely and look at the retractor for scorching or a locked reel. If the belt fired, the diagnosis is over and the repair is the belt.
If the belt is intact, disconnect the battery, let the system sit at least three minutes, then unplug the connector at the belt assembly and inspect both halves for bent, pushed out, or corroded terminals. Follow the No. 2 floor wire under the carpet and sill trim for crush damage and for the green tint water leaves on a connector. Related codes are B1900 for a short in the same circuit, B1906 for the same open on the left side, and B1801 for an open in the driver airbag squib circuit.
Related codes: B1900, B1906, B1801
Keep reading:
Code scope: Toyota, Lexus, Scion. The same number can mean something different on another manufacturer, so match the code to your make before you act on it.
A reset clears stored crash data. A replacement part fixes a hardware fault. We have read these codes since the nineties and will tell you which one you are looking at.
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